@Curt I just saw your post. If you haven't bought the Interlink yet, I have one which I can sell you, with cabling, for a lot less than the $200 they want at the Humminbird site. I upgraded but had used it to connect two 788c head units.
I had my dealer do the install so I have no tips there, but it can't be too hard. There are only 3 inputs to the interlink: 2 head units and one GPS antenna. I think there are instructions on the Humminbird site. Send me a PM if interested and we will work out the details.
Jake